Patent GB-230563-A (GB)
Title: Improvements in telephone testing systems
IPC: H04M1/24
Filed: 1923-12-20
Abstract: 230,563. Western Electric Co., Ltd., (Western Electric Co., Inc.). Dec. 20, 1923. Measuring transmission equivalent of telephone lines and efficiency of telephone instruments. - Determinations of the efficiency of subscriber's telephone instruments or of transmission lines such as subscribers' loops, are made without removing the instruments from the substation, by transmitting over the loop from the central office definite direct and alternating currents, and taking observations of current strength and voltage when using the apparatus under test, the results obtained being compared with a standard. For example, in testing a subscriber's transmitter T, a resistance 1 equal in value to the direct-current resistance of a standard transmitter when subjected to standard agitation is connected across the line 5, 7 and the direct current transmitted over the line from a battery 3 is adjusted by means of a resistance 2 to give a suitable reading of an ammeter A1. The resistance 1 is then replaced by the transmitter T which is subjected to standard agitation by means of an acoustic coupling to a standard receiver R supplied with alternating-current from a source 11, the voltage supplied to the line being adjusted by means of a resistance 10 to give a desired reading of the voltmeter V1. The change in the reading of the ammeter A1 then gives a measure of the alternating voltage generated by the transmitter T, and by comparing this with the value obtained with a standard transmitter subjected to the same conditions, a measure of the efficiency, which may be converted into miles of standard cable is obtained. The circuits employed for testing a receiver are similar to those shown, a standard receiver and the subscriber's receiver being in turn coupled to the transmitter T whilst the alternating voltage applied is varied by means of resistance 10 until the outputs of the receivers, as determined by the reading of the ammeter A1, are the same. The ratio of the two readings of the voltmeter V1 then gives a measure of the efficiency of the subscriber's receiver. The efficiency of the subscriber's loop 5, 7 is determined by comparing the voltages required to produce the same effect in the receiver R and transmitter T when the energy is supplied through the whole of the coil 13 the impedance of which is very high compared with that of the loop, and when supplied through only a few turns of the coil 13, the impedance of these turns being equal to that of a standard subscriber's set and comparable with the impedance of the loop itself. In order to obtain more accurate directcurrent readings, a second more sensitive ammeter A2 may be employed which covers only a small range and is adjusted to register over the range required by means of a shunt-connected battery 17 and resistance 18. A tube 16 may enclose the air column between the coupled transmitter and receiver. Preferably, the source 11 generates a band of frequencies corresponding in amplitude and range with the normal speechwave components. In a modified circuit, particularly for use with loops of varying lengths, additional impedances are inserted in series with the supply circuit 10, 11 and the voltmeter V1 is connected directly across this circuit.